The monolithic crystal filter was invented by William D. Beaver and Roger A. Sykes and patented in Canadian Pat. No. 863,480 granted Feb. 9, 1971 to Western Electric Company, Incorporated, U.S.A. This patent is generally useful in understanding the theory and operation of such devices. More recently, practical polylithic filter devices, that is filters utilizing a plurality of monolithic crystals, have been patented. U.S. Pat. No. 3,676,806 issued July 11, 1972 to Henry T. Orchard and Desmond F. Sheahan, for example, discloses such practical device.
In essence, the basic 2-pole monolithic crystal is a specifically dimensioned monolithic slab of piezoelectric material having electrodes for two resonators deposited on its surfaces. It performs a filtering function because of the intra-crystal coupling between the resonators. The coupling between resonators is to a large extent determined by the interelectrode spacing, often a critical factor difficult to control
It is the difficulty of controlling the critical coupling in a monolithic crystal, for example, that does not permit practical predistorted filter designs. In a paper by R.J. Byrne entitled: "Monolithic Crystal Filters", Proceedings of the 24th ASFC (Annual Symp. on Frequency Control), pp 84-92, April 1970, the most critical parameter in the manufacture of these filters is found to be the coupling between resonators.